1. Field of the Invention
The present invention relates to an image forming apparatus utilizing an electrophotographic system, and in particular, to an image forming apparatus such as a copying machine, a printer or a facsimile unit.
2. Description of the Related Art
Heretofore, there have been widely well-known an image forming apparatus mainly utilizing an electrophotographic system, such as a copying machine or a printer. Monochromatic image forming apparatuses as well as full color image forming apparatuses are now widely and commercially available. Further, as the image forming apparatus of the electrophotographic type has been used in various fields, demands for the image quality thereof have been higher.
It is noted that a degree of smoothness of an output image may be one of the factors which can determine a quality of an image, that is, a degree of glossiness of a full color image. For example, in order to comply with the above-mentioned demands, Japanese Patent Laid-Open No. S64-35452 or Japanese Patent Laid-Open No. H05-216322 discloses an image forming method of forming a color image bearing product having a high degree of glossiness, in which a recording medium coated thereover with a transparent resin layer, as a glossy layer, made of thermoplastic resin is transferred thereon with an unfixed color toner image made of thermoplastic resin, and then is heated so as to fuse the color toner image in order to fix the same.
In the above-mentioned image forming method, a belt fixing unit has been proposed for desired fixation. For example, a belt fixing unit disclosed in Japanese Patent Laid-Open No. H04-216580 or Japanese Patent Laid-Open No. H04-362679 has such a configuration that a recording medium carrying thereon a unfixed toner image is pressed and heated by a fixing belt made of a heat-resistance film having a smooth surface, then is cooled while it is held in close contact with the fixing belt in order to solidify the toner image. Thereafter, the recording medium to which the toner image is fixed is separated from the fixing belt. As a result, the toner image is fixed while being embedded in the transparent resin layer of the recording medium and while the transparent resin layer on the surface of the recording medium and the toner image are both fixed copying the surface configuration of the belt. Accordingly, the entire surface of the recording medium becomes smoother. Thereby, it is possible to obtain a color image which is excellent in glossiness.
Further, a recording medium having a resin layer as a glossy layer is proposed in Japanese Patent Laid-Open No. 2003-084477 which thus discloses an electrophotographic image transfer sheet coated thereover with a resin layer made of, for example, thermoplastic resin, as a main component, having a glass transition temperature of not higher than 85 deg. C. and having a thickness of about 20 μm.
However, as stated above, the glossy image forming method in which a toner image is formed on a recording medium having the glossy layer on its surface, and is fixed by the belt fixing unit so as to obtain a glossy image medium would possibly causes the following problems:
1) First, should the original surface of the glossy layer of the recording medium be unsmooth, the belt fixing unit should apply a heat and a pressure which are required for enabling the glossy layer of the recording medium to have a required smooth surface. That is, the lower the degree of smoothness of the surface of the glossy layer of the recording medium before image formation, higher heat and pressure are required.
However, should the heat value and the degree of pressure of the belt fixing unit be increased, the toner image and the resin layer as the gloss layer would be subjected to heat and pressure which are accordingly high. As a result, the heat and the pressure which are excessive, would cause the toner to be excessively fused so as to be offset onto a fixing belt or to flow over the surface of the recording medium, resulting in a problem of blurring of the toner image, and further, to deviate the toner image together with the resin layer as the glossy layer of the recording medium, resulting in a problem of disturbance of the toner image.
Thus, in the case of a low degree of smoothness of the surface of the glossy layer of the recoding medium before image formation, it is self-explanatory that there is a limitation in increase of heat and pressure of the belt fixing unit for enabling the surface of the glossy layer to have a desired smooth surface.
2) Accordingly, although it is preferable to allow the surface of the glossy layer of the recoding medium to be previously smooth before using, the recoding medium of such kind would possibly cause an occurrence of a problem of double feeding of the recording medium when one sheet of the recoding medium which has been stacked in a paper feed portion is separated from the other sheets of the recording medium and is fed.
That is, in the case of fixing a unfixed toner image on a recording medium having at its one surface a glossy layer, there have been raised the above-mentioned technical problems.